1. Field of the Invention
The present invention relates generally to systems for maintaining a generally constant level of fluid within a vessel and more particularly to systems for supplying vapor to a chemical process by introducing a carrier gas into a fluid column of vaporizable liquid.
2. Background Information
A common technique used in vapor generating systems for delivering chemical vapor to a process chamber is to force a carrier gas bubble through a chemical fluid in a bubbler and then to deliver the resulting vapor from the bubbler to the process chamber. Traditional bubblers, including those utilized in presently available automatic refill systems, rely on relatively large fluid volumes to intrinsically compensate for deviations in fluid level which can negatively effect the resulting vapor concentration. Since vapor sources in the fiber optics and semiconductor industries are often hazardous fluids, there has been an increasing focus on the occupational safety and health concerns resulting from use of such fluids. This has resulted in reducing the maximum allowable volumes of many of these fluids within the work place. It is therefore desirable to reduce the required fluid volume at the point of vapor generation without compromising vapor concentration control.
Typically a bubbler container is comprised of a single vessel which holds an expendable volume of vaporizable fluid. A carrier gas such as hydrogen, helium or nitrogen is introduced at the lower level of a fluid column, travels up through, and exits the fluid surface into a head space. As the carrier gas passes through the fluid column it becomes entrained with vapor which results in a corresponding reduction of the fluid volume. This reduction of the fluid level in the bubbler container may be significant for several reasons. For example, the vaporization efficiency and overall vapor concentration uniformity are both affected by the fluid level and are both important elements which may affect the strict tolerance requirements of the process application. In addition, the physical fluid column in the bubbler not only determines the carrier gas contact time and resulting bubble geometry but also represents the mass to which thermal energy is either added or extracted. It also defines the head space present above and within the bubbler container which has been found to negatively effect the vapor concentration and ultimate bubbler performance when not optimized.
Inasmuch as vapor extraction from a fluid volume results in depleting the fluid volume of a bubbler, causing variations in vapor concentration, a means of replenishing this fluid is desirable. Some methods include manually replacing the bubbler ampule once the volume of fluid reaches a predetermined minimum acceptable level. Other manual methods rely on an auxiliary supply of fluid to replenish the bubbler during intermittent periods of non-use. Although such methods can result in reducing many of the concerns associated with prior art expendable bubblers, such as reducing the risk of contamination during ampule replacement or any necessary fluid replenishment, these systems typically remain idle until an interruption in vapor extraction provides a refill opportunity. With many of the advanced processes running for long periods of time before a refill opportunity is presented, the fluid level may descend considerably resulting in less than optimum vapor delivery efficiency. Although there are techniques which can be employed to compensate for the influences of a constantly descending fluid volume, such as intermittent refill in between process runs, such techniques can be complex and costly. In any event, such techniques do not satisfy the level of control achieved by the present invention.
In addition to manual replenishment of fluid, automatic bubbler refill systems are also available. However, such systems typically employ float coupled electronic devices, such as level controllers, to control the replenishment of fluid in the bubbler. Such devices are prone to failure and are generally the most common failure mechanism in the system. Other types of fluid level sensors such as optical, load cell monitoring of the contents, and resistance probes have been employed. However, the use of such devices can be costly, prone to error, and with many of the fluids being flammable, represent ignition sources if not properly rated and maintained.